Chapter Five Part 6: Exceptions

1. Chapter FivePart 6: Exceptions

2. Exceptions • Hardest part of processor design: hard to get right, hard to make fast. • Exceptions: an unexpected event from within the processor. • Example: arithmetic overflow. • Interrupt: an event that causes an unexpected change in control flow, comes from outside of the processor. • Example: I/O device interrupt • Some architectures do not distinguish between these two. • MIPS: • Exception: any unexpected change in control flow (both internal and external) • Interrupt: an unexpected change in control flow that is caused by an external event.

3. Exceptions • Examples: • This chapter: control implementation for • Undefined instruction • Arithmetic overflow • Exceptions often on the critical timing path of a machine.

4. Exceptions • Actions: • Save the address of the offending instruction in the exception program counter (EPC) • Transfer control to the OS at some specific address • After OS handles error • terminate program or • continue execution (using EPC)

5. Exceptions • OS must also know reason for exception. Two methods: • Use a status register (called the cause register in MIPS). • Holds a field to indicate reason. • A single entry point for all exceptions • OS decodes the status register to find the cause • Vectored Interrupts. • The address to which control is transferred is determined by the cause of the exception.

6. Exceptions • MIPS (uses status register). Need 2 new registers: • EPC. 32-bit register used to hold the address of the instruction that caused the interrupt • Cause. 32-bit register used to record the cause of the exception. Don’t need all bits. • Undefined instruction = 0 • Arithmetic overflow = 1

7. Exceptions • Need 2 control signals to cause the EPC and Cause registers to be written: • EPCWrite • CauseWrite • Need 1-bit control signal to set the low-order bit of the Cause register • (IntCause).

8. Exceptions • Must write the exception handler address into the PC. Assume this is C0000000 • PC is fed by 3-way mux. Must change to 4-way. Controlled by • PCSourceS • 4th source hardwired to C0000000.

9. Exceptions • Cannot write PC into EPC (‘cause PC contains PC+4 or instruction following the bad instruction) • Use ALU to subtract 4 from PC and write to EPC • Already have 4 hardcoded. • So connect data write port of EPC to ALU output

10. Exceptions

11. Exceptions

12. Exceptions • Control for detecting exceptions • Undefined instruction: detected when no next state is defined from state 1 for the op value. • Define next state value for all op values not defined as state 10. • FSM uses the word other. • Arithmetic overflow: Have a signal called overflow from the ALU. • Use in FSM to specify an additional possible next state for state 7 • Real machine: many different exception-causing events. • Must include these and still keep control small and fast.

13. Exceptions

14. Exceptions • Some classes of exceptions require that the state in the FSM not change. • Discussed in chapter 7. • Require us to prevent instruction from changing the machine state. • Makes control complex.

15. Exceptions • Vectored Interrupts. The address to which control is transferred is determined by the cause of the exception. • Example. Might define the following: • The OS knows the reason for the exception by the address at which it is initiated. • Addresses separated by 32 bytes or 8 instructions. • OS must record the reason for the exception and either handle the exception or branch in 8 instructions.

16. The Big Picture